Marvel's superhero ensemble, which stars Robert Downey Jr, Chris Hemsworth and Scarlett Johansson, pulls in more than £8 million to see off competition from comedy sequel American Pie: Reunion.
Zac Efron's war drama The Lucky One debuts at number three ahead of the 3D re-release of Beauty and the Beast, while Jason Statham's action thriller Safe rounds out the top five.
Last week's runner-up Salmon Fishing in the Yemen falls down four places to number six, The Hunger Games is at number seven and Aardman's The Pirates! lands at eight.
Silent House is the only other new release to make the top ten. Elizabeth Olsen's single-take horror movie debuts at number nine with £214,731.
The UK box office top ten figures in full:
1. (1) Avengers Assemble - £8,121,916
2. (-) American Pie: Reunion - £6,334,539
3. (-) The Lucky One - £1,159,435
4. (-) Beauty and the Beast 3D - £685,024
5. (-) Safe - £679,716
6. (2) Salmon Fishing in the Yemen - £641,051
7. (3) The Hunger Games - £346,807
8. (4) The Pirates! In an Adventure with Scientists - £318,039
9. (-) Silent House - £214,731
10. (7) Mirror Mirror - £207,319
Watch an Avengers 'Threat' video featurette below: